1. Field of the Invention
This invention pertains generally to nickel-base superalloys which are capable of being undirectionally solidified to form articles of manufacture having a composite microstructure of an aligned fibrous monocarbide eutectic reinforcing phase embedded in a nickel-base superalloy matrix. More particularly, this invention pertains to monocarbide reinforced nickel-base superalloy eutectics which are substantially phase stable and have improved high temperature stress-rupture strength with improved resistance to the formation of surface nucleated carbides during unidirectional solidification.
2. Description of the Prior Art
Monocarbide reinforced nickel-base superalloy eutectics of the predominantly TaC reinforced type described, for example, in U.S. Pat. Nos. 3,904,402, 4,284,430 and 4,292,076 to Smashey, Henry and Gigliotti, Jr., et al. respectively, which are herein incorporated by reference, are primarily designed for use as unidirectionally solidified anisotropic metallic bodies in the form of vanes and rotating blades in aircraft gas turbine engines. Such aircraft engines present operating environments which require, for example, cyclic oxidation resistance, hot corrosion resistance, and high strength at high temperatures for such parts as vanes and rotating blades.
An undesirable feature of many monocarbide reinforced eutectic superalloys, including the preferred and more preferred alloys of U.S. Pat. Nos. 4,284,430 and 4,292,076, is the propensity for formation of microstructural instabilities after long-time exposure at elevated temperatures. Microstructural instabilities are platelet-like or globular precipitates of phases, i.e., M.sub.6 C or .sigma., which result from a reaction between elements in the superalloy matrix, notably tungsten ad molybdenum, and the tantalum carbide fibers or whiskers. The precipitated phase is considered undesirable since it has the potential to decrease strength by depleting the matrix of strengthening elements, is a potential site for early crack initiation in its platelet-like morphology, and is a potential site for early onset of oxidation due to its low aluminum and high refractory metal content.
Another undesirable feature of such eutectic superalloys, particularly in the form of hollow air cooled airfoil castings, is the presence of blocky, surface nucleated carbides. In such superalloys, those carbides form an essentially continuous film on all cast surfaces, and extend to a depth typically of about 6 miils (150.mu.) into the casting. Nucleation of such carbides on mold or core walls occurs in the liquid ahead of the advancing solidification growth front during unidirectional solidification. Surface carbide dimensions depend on the temperature gradient in the liquid and the growth (solidification) rates, since those carbides continue to grow until the advancing front consumes all the liquid available to the carbide. Those surface nucleated carbides are undesirable in that they reduce strength, i.e., in some thin-wall hollow airfoil sections those carbides can reduce load-bearing area by 40%. Furthermore, those carbides can act as crack nucleation sites, thus detrimentally affecting fatigue life and resistance to failure by stress-rupture. Normally, such surface carbides must be removed by expensive mechanical machining processes in order to render the castings suitable for end use in aircraft gas turbine engines.
While the alloys of U.S. Pat. Nos. 4,284,430 and 4,292,076 are prone to the routine formation of large amounts of surface nucleated carbides, others, such as the alloys of U.S. Pat. No. 3,904,402, are not. There are a great many differences in composition among those alloys. The elements whose amounts vary greatly among those alloys are Ta, V, Mo, W and Cr. For example, the alloys of U.S. Pat. Nos. 4,284,430 and 4,292,076 contain Mo, large amounts of Ta and no V while the alloys of U.S. Pat. No. 3,904,402 contain V, lesser amounts of Ta and no Mo. Additionally, it is known that TiC and CbC reinforced eutectic alloys are also prone to the formation of large amounts of surface nucleated carbides.
Thus, although the eutectic superalloys described above represent significant advances in the metallurgical arts, there is room for further improvement. Specifically desirable would be eutectic superalloys which are phase stable and have improved resistance to the formation of surface nucleated carbides, but which also have sufficient transverse ductility, cyclic oxidation resistance, hot corrosion resistance and high temperature strength properties, including stress-rupture strength, to render them acceptable for use in aggressive environments, especially those found in aircraft gas turbine engines.